> From: "Joe Trombino" <w2kj@earthlink.net>
> To: <amsat-bb@AMSAT.Org>
> Subject: [amsat-bb] pre-amps, etc
> Date: Mon, 19 Feb 2001 09:52:52 -0500
> I've been following with great interest the thread about
> using pre-amps and how not to blow them up.
>
> With this in mind, I wonder if it is possible to come out of
> the Drake 2880 at 144Mhz into a two meter converter with a
> 28Mhz output into a general coverage or even ham band
> receiver. This would eliminate accidental transmitting into
> the pre-amp/downcoverter set-up for 2400Mhz.
>
> I'm new to this frequency range (2400Mhz) so please forgive
> the un-educated ramblings of a newbie:-)
>
> 73, Joe W2KJ
Dear Joe,
Your basic idea is sound. Many of us will be doing exactly that, but
for a much different reason.
OK, you have two problems here:
1. Protecting the converter(s)
2. Generating the uplink signal
The two are related in a way. Lets look at the 1.2Ghz uplink problem
first. Lets say your like me and don't want to spend big bucks on a
specialized new sat rig with a 1.2Ghz transmitter. The alternative is
to use a transverter. The usual approach is to use a 2M IF, that is to
use your existing 2M multimode rig as the excitor for the 1.2Ghz transverter.
This produces the related problem, when your transmitting on 2M, you
cannot use it for a receiving IF! Oops!
Solution? Use a first IF for the 2.4Ghz receiving set up other then one
related to the uplink transmitter/transverter. This also precludes 70cm
(the third harmonic of 2M). That's OK because the Drake can be made
to work from 70cm down to below 2M. Its all a matter of what you do to
the output filter.
Lets say we pick a band that is not harmonically related and is useful
in itself. I decided on 222Mhz. So now we convert from 2.4Ghz down to
222Mhz. And from there to 28Mhz! This allows us to use our "good"
HF receiver for 2.4Ghz reception. Chances are that it is a much better
receiver then the usual VHF multimode rig most of us have. This is
a good thing!
Now, we still have a transceiver connected to a converter. In this case,
we blow up the 222Mhz converter instead of the Drake. Not to fear,
we add a squencer to the receiving system, and the problem is solved!
You accidently key the transmitter, and before any damage can be
done, a relay switches the HF rig to dummy load or HF antenna.
No problem! You can even modify the squencer so it inhibits the HF
rig from transmitting when the converters are turned on! This is what
I did as well.
Squencers are simple and easy to install into the station system.
Yes you have to do a minor modification to your radios PTT circuit.
This is no big deal at all. Anyone can do it in an hour. The same
squencer can also hard key your PA amps as well. This is a vastly
better way to do things than RF sensing.
You can find a squencer schematic in any recent edition of the ARRL
Handbook. Let me know if you have any questions.
--73-- David WA0AUQ
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